How Container Rain Forms — The Dew Point Cycle
The mechanism of container rain is straightforward:
- Cargo is packed in a warm, humid environment — a South African coast or inland warehouse, often in summer. The air inside the container at the time of sealing contains a significant amount of water vapour.
- Additional moisture is released inside the container from wooden pallets, cardboard packaging, and the cargo itself as they off-gas during the voyage.
- As the vessel moves from warm loading ports into cooler ocean temperatures — or when the container passes through cooler nights or enters refrigerated storage — the air inside cools.
- When the air temperature drops below the dew point — the temperature at which the air is fully saturated with water vapour — the excess moisture condenses out of the air as liquid water.
- This condensation forms on the coldest surfaces in the container, typically the metal ceiling and walls. When enough water accumulates, it drips onto the cargo — hence ‘container rain.’
- As the container warms again, the cycle reverses — liquid water evaporates back into the air, raising humidity again, and the cycle repeats. Each condensation cycle deposits more moisture on cargo surfaces.
What Makes Container Rain Worse
- High pallet moisture content — freshly cut or improperly dried timber pallets can contain significant moisture that continues off-gassing throughout the voyage
- Humid loading ports — Durban, Mombasa, Lagos, Singapore, and other tropical or coastal ports have high ambient humidity at the time of container stuffing
- Long voyages — the longer the voyage, the more condensation cycles occur. Voyages from South Africa to Asia or the Americas can last 20 to 35 days with multiple dew-point crossings
- Equatorial crossings — voyages that cross the equator experience the most extreme temperature swings, typically from warm tropical sea temperatures to cooler open-ocean conditions north and south
- Dense cargo with high inherent moisture — agricultural produce, wood products, and processed food all carry significant internal moisture that is released inside the container during the voyage
- Inadequate ventilation — fully sealed containers with no airflow have no mechanism to equalise humidity with the external atmosphere
The Financial Impact of Container Rain
Container rain is one of the leading causes of cargo claims globally. Common damage categories include:
- Mould growth on food products, textiles, leather goods, and paper packaging — often rendering cargo unsaleable on arrival
- Rust and corrosion on metal surfaces, machinery, tools, and steel products — including corrosion under protective packaging that is only discovered at destination
- Carton collapse — waterlogged packaging loses structural integrity, causing stacked loads to collapse, damaging cargo and creating safety hazards during unloading
- Label damage — adhesive paper labels peel and separate when wet, causing identification problems at customs and in distribution
- Caking of powdered products — powders, granules, and pigments absorb moisture and form hard lumps, making them unusable or requiring reprocessing
DunLash Ultra — How It Prevents Container Rain
DunLash Ultra 750g container desiccants absorb the water vapour from the container atmosphere before it can reach dew point and condense. By reducing the humidity inside the container, DunLash Ultra prevents the condensation cycle from starting — eliminating container rain at its source rather than managing the damage after it occurs.
| DunLash Ultra Performance | Result |
|---|
| Absorbs up to 250% of its weight at 30°C / 80% RH | Removes moisture from the container air before dew point is reached |
| Absorbs up to 300% at 30°C / 90% RH | Effective even in high-humidity loading environments |
| Absorbs up to 150% at 28°C / 70% RH | Continues absorbing as voyage progresses and conditions change |
| 750g CaCl2 per bag with hanger fitting | Simple installation — bags hang from container ceiling rings |
| Operating range -5°C to 40°C | Effective across the full temperature range of standard sea voyages |
Dosage Guide — How Many Bags Per Container
The minimum recommended dosage for standard GP containers:
- 20ft (6m) GP container — minimum 4 x DunLash Ultra 750g bags
- 40ft (12m) GP container — minimum 8 x DunLash Ultra 750g bags
Increase the dosage when any of the following apply:
- High pallet moisture content — freshly cut timber or undried pallets release significantly more moisture
- Voyage crosses the equator — greater temperature swings create more intense condensation cycles
- Voyage duration exceeds 21 days — more dew-point cycles require greater total absorption capacity
- Cargo has inherent high moisture — agricultural produce, wood products, fresh-processed food
- Loading port has extreme humidity — Durban, Mombasa, Lagos, Singapore, Shanghai
Specific Applications by Cargo Type
| Cargo Type | DunLash Desiccant Recommendation |
|---|
| Fresh fruit, citrus, stone fruit, grapes | DunLash Ultra in container · High dosage recommended due to fruit moisture release |
| Grain, wheat, maize, bulk cereals | DunLash Ultra · Dosage based on grain moisture content and voyage duration |
| Dried fruit, nuts, confectionery | DunLash Ultra · High dosage recommended |
| Coffee and cocoa | DunLash Ultra, FCC approved for cocoa · Contact DunLash for voyage-specific dosing |
| Tobacco and leaf products | DunLash Ultra · Higher moisture sensitivity requires increased dosage |
| Wine and beverages | DunLash Ultra · Helps prevent label damage and capsule corrosion from container rain |
| Packaged food products | DunLash Ultra in container |
| Metal and machinery | DunLash Ultra · Prevents rust and corrosion during long voyages |
Why Choose DunLash for Container Rain Prevention
DunLash supplies moisture control and cargo protection products for South African exporters, freight forwarders, agricultural producers, packhouses, logistics companies, and food cargo operators. Our solutions help protect cargo from condensation, container rain, mould growth, packaging collapse, corrosion, and moisture-related claims during road, rail, and ocean freight.
DunLash can assist with desiccant dosage recommendations based on cargo type, container type, voyage duration, route profile, pallet moisture, and specific export requirements.
